Acoustic backscattering characteristics of swimbladdered juvenile fish derived from cage experiments

Acoustic target strengths of captive juvenile fishes of three species were measured at four frequencies using fish cage method. Measurements were conducted on a barge constructed at a fishing port for th e experiments, Measured fish were sea smelt Hypomesus pretiosus japonicus ( Brevoort), goldeye rockfish Sebastes inermis (Cuvier), and purple puffer Ta kifugu porphyreus (Temminck et Schlegel). They were caught with a miniature purse seine and maintained in fish breeding tanks before the experiments. Their body lengths ranged from 30 to 65 mm. Each species was classified int o three size classes. Specimens were maintained in a cubical cage of 0.5 m each side made of lace cloth during measurement. The echo signals from enca ged fish at 25, 50, 100, and 200 kHz were recorded with a data recorder. Ec ho signals were later digitized and processed by squared integration to cal culate the target strengths. The results show that, (1) The backscattering strength of encaged juveniles indicates a linear density dependence for most sample sets, (2) The normal ized target strengths TScm of juveniles ranged from -66 to -55 dB, which is maximum 10 dB higher than that of adults, (3) Target strength changed by t he frequency, that is, the TScm for L/lambda less than or equal to 2 was ro ughly 10 dB higher than the TScm for L/lambda greater than or equal to 2.